1. Field of the Invention
The present invention relates generally to a method for controlling a dynamic tracking head and more particularly to a method for controlling a dynamic tracking head of a digital video tape recorder for recording and reproducing a segmented track type format.
2. Description of the Prior Art
According to a prior-art helical scan type analog video tape recorder for recording an analog video signal by a rotary magnetic head, a slanted recording track Tk is formed on a magnetic tape. One field of the analog video signal is recorded on one track starting at the beginning portion as shown in FIG. 1. In FIG. 1, Vh represents the moving direction of the magnetic head and Vt represents the transportation direction of the magnetic tape T.
When a video signal is reproduced from the above-mentioned magnetic tape T, in (1) a variable tape speed playback mode such as a still picture playback mode, (2) slow motion playback mode or (3) a high speed playback mode, the tracing locus of the magnetic head intersects the recorded track, lowering the level of a reproduced RF signal, the generation of a cross-talk component and so on.
To solve the above-mentioned problems, a so-called dynamic tracking technique is employed. According to this dynamic tracking technique, the magnetic head is mounted on a rotary head drum through, for example, a piezo-electric bimorph plate. A control signal is applied to the piezo-electric bimorph plate to displace the magnetic head in the widthwise direction of the magnetic tape so that even in the variable tape speed playback mode, the magnetic head precisely traces a predetermined track.
In the above-mentioned dynamic tracking technique, the displacing direction of the magnetic head is opposite at the starting portion and the ending portion of the track. When the magnetic head moves (or jumps) from the end portion of the current predetermined video track (field) to the starting portion of the next predetermined video track (field), the polarity of the control signal is inverted thereby moving the magnetic head from one end portion of its range of displacement to its other end portion. This movement of the magnetic head is performed outside of the tape wrapping range and during a period (blank period) in which the magnetic head rotates without contacting the magnetic tape. U.S. Pat. No. 4,287,538 discloses an apparatus for controlling the above-mentiond movement of the magnetic head, or the track jump of the magnetic head.
Recently, International Standard (D-1 Standard) for a digital video tape recorder for digitally recording a component type video signal (luminance signal Y and difference chrominance signals R-Y and B-Y) has been determined.
FIG. 2 is a diagrammatic view of a section of a magnetic tape T, illustrating a slanted recording track according to this D-1 standard. The slanted recording track is called a "program track" and is comprised of (1) two segmented video tracks or sectors (half tracks) Sv in which digital video data are recorded, and (2) four segmented audio tracks or sectors Sa in which digital audio data are recorded. Further, a cue/audio track Tka, a servo control track Tkc and a time code track Tkt are formed on the magnetic tape T in the longitudinal direction thereof.
The above-mentioned track pattern is substantially common to a 525/60 system having 525 horizontal lines and a field frequency of 60 Hz (nominal frequency) and a 625/50 system having 625 horizontal lines and a field frequency of 50 Hz.
The D-1 system format handles much data. As a result, a 4-channel processing system is used in order to reduce a transmission rate for the data. As, for example, shown in FIG. 3, four magnetic heads H1, H2, H3 and H4 are paired two by two to provide head pairs HA (including H1 and H2) and HB (including H3 and H4). The head pairs HA and HB are mounted on a rotary head drum RD with an angular destance of 180 degrees. The magnetic tape T is wound around the rotary head drum RD over a tape wrapping angle of 260 degrees. Video signal data are recorded on recorded tracks of tape wrapping angles of 120 degrees between points A and B and between points C and D. Audio data is recorded on the recorded track at a tape wrapping angle of 20 degrees between points B and C.
The two slanted recording tracks Sv shown in FIG. 2 are sequentially formed on the magnetic tape T by the above-mentioned head pairs HA and HB.
In the case of the 525/60 system format, one field of video data is recorded on 20 segmented video tracks S1 to S20 corresponding to 10 slant tracks as shown by the solid lines in FIG. 4.
In the case of the 625/50 system format, while the data transmission rate is the same as that of the 525/60 system format, the duration of the field cycle thereof is long so that one field of video data is recorded on 24 segmented video tracks S1 to S24 (four segmented video tracks S21 to S24 are additionally provided)/ corresponding to 12 slant tracks.
In both cases, the four segmented video tracks S1 to S4 cross-hatched in FIG. 4 respectively correspond to the four magnetic heads H1 to H4 shown in FIG. 3, and form one segment set.
The recording of one field of video data is started at the segmented video tracks S1 and S2 (the upper side of FIG. 4) which start from substantially the central, portion of the magnetic tape along its width and which ends at the end portions of the segmented video tracks S19, S20 or S23, S24 at the nearby-portion of the servo control track Tkc (see FIG. 2).
When the digital video tape recorder having the D-1 type format reproduces the recorded video signal in the variable tape speed playback mode, there then is presented the following problem of data dropout.
As earlier noted, according to the D-1 type format, the starting portion and the ending portion of one field are both located at the central portion of the magnetic tape and the length of one field in the longitudinal direction of the track is short. For this reason, it is mechanically difficult for the magnetic head to jump from the end portion of the current predetermined field to the starting portion of the next predetermined field during a period in which the magnetic head also passes this short distance (the segmented audio tracks) by the above-mentioned dynamic tracking technique. As a result, reproduced data at the starting portion of the field, for example, reproduced data of 3 segmented video tracks (half tracks) can not be obtained sufficiently causing the image quality of the reproduced picture to be deteriorated.